The feasibility of probing high resolution mutagenesis at the thymidine kinase (TK) locus in L5178Y mouse lymphoma cells will be examined (alpha) to more thoroughly evaluate the hypothesis that small sigma colonies in the bimodal distribution of L5178Y cells mutants are the result of more extensive damage to the region of the TK genes (chromosome 11b) and that large (lambda) colonies result from point mutations to the TK gene, and (b) to characterize omega and lambda mutants arising in L5178Y mouse strains 3.7.2C, LY-S1, and LY-R83 grown in folic acid-deficient medium then exposed to ethylmethane sulfonate. The phenotype of mutant cultures at the TK and galactokinase (GK) loci will be characterized; high resolution banded chromosomes (number 11) will be analyzed, and in situ hybridization of radioactive probes of TK and GK cDNA to the high-resolution banded chromosomes will be initiated. In Phase II these approaches will be continued and the mutants will be characterized at the molecular level.